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my little chihuahua is 7
my little chihuahua is 7 years old and is heart worm pos i have her on trifexis vet told me the heart worms wood not get any worse but she has a bad cough more like a hack i take her to the vet and they give her a shot not sure what it is but it stops the hacking right then it use to last 3 months now its once a month if that long this shot is just not working anymore this has been going on for years and every time she gets the shot its 45 bucks witch if it wood last longer than a month i wood not mind it.can anyone help me and my little bonnie with any advice